Porous composite filler compositions

ABSTRACT

A composite filler comprising thermally processed porous inorganic mixed particles of silica and at least one heteroparticle selected from the group consisting of zirconia, hafnia, or yttria and a polymer occupying the pores of the porous inorganic mixed particles, wherein the porous inorganic mixed particles are thermally processed at a temperature of from 650 to 900° C., as well as a dental restorative comprising a resin and a composite filler, and optionally other fillers, wherein said resin has a refractive index that increases upon curing, and wherein the opacities of the both uncured and cured restorative are less than 45.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.15/462,836, filed on Mar. 18, 2017, the contents of which are herebyincorporated by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to polymeric composites comprisinginorganic fillers and organic, or organometallic, polymers. Theinvention relates to filler compositions that are used in thepreparation of inorganic-organic polymeric composites, and, inparticular, light curable composites. Still further, the inventionrelates to composite fillers that are used in dental applications, suchas tooth restorations, for example, cavity fillings, adhesivecompositions, veneers, crowns, bridges and teeth replacements.

BACKGROUND OF THE INVENTION

Inorganic-organic polymer composite materials are used in a wide varietyof applications including structural materials, high performancecomposites, optical components, aerospace, biomedical implants anddental applications. Generally, composites are employed whereperformance requirements are demanding and not easily fulfilled withtraditional structural materials. For example, inorganic materials, suchas glass, ceramic and stone, are very hard, scratch resistant and evensometimes transparent (e.g., glass), but suffer from the fact that theyare very heavy and brittle. Polymers, conversely, are light and durable,but have poor hardness, abrasion and wear resistance. Composites, madefrom the combination of inorganic materials and polymers, may haveproperties that lie in between, providing materials that aresimultaneously strong but lightweight, hard but flexible, abrasionresistant and durable.

In order to achieve such properties, in practice, hard inorganicmaterials are mixed into polymers, or polymer precursors, monomersand/or oligomers, referred to as resins, and the mixture is then curedto form a composite. Hereafter, inorganic addenda are referred to asperformance additives. Performance additives are an extremely importantcomponent of coatings and composite formulations. They impart a widevariety of properties to the end products including strength andtoughness, scratch and mar resistance, UV absorption, opticalproperties, anticorrosion, and biocompatibility (for medical basedcoatings). Typical performance addenda are comprised of inorganic metaloxides, such as silica, titania, alumina, and zinc oxide; they may becategorized according to their size: micron-sized (0.2-100 μm) ornano-sized (1-200 nm).

There are several problems or difficulties generally experienced inmixing performance additives into polymers. First, polymers or polymerprecursors may be viscous and the addition of performance materials onlyincreases the viscosity and limits the loading of material that may beachieved, and creates difficulty in handling, molding and crafting thecomposite into an article of commerce. Second, inorganic performancematerials generally have a high surface energy compared to resins, andthe mismatch in the interfacial energy may cause the inorganic materialsto agglomerate and/or aggregate, making a homogeneous dispersiondifficult or impossible to achieve. This problem is particularly acuteif the particle size of the performance additive is small, especially inthe case of nanomaterials, i.e., materials with a particle size between1 to 200 nm.

The polymer industry is transforming from composites that arepolymerized, or cured, using heat (thermal set polymers) to those thatare cured using ultraviolet or visible light, or low energy electrons(UVEB). UVEB curable resins offer tremendous energy and waste savings tothe coatings and composites industries because they are polymerized(cured) directly with light and also because they generally do notcontain volatile diluents, such as solvents or carriers that may beconsidered hazardous air pollutants. UVEB curing is far more energyefficient, since it overcomes the thermal loss that is prevalent inconventional thermoset coating systems. Ironically, the fundamentaladvantages of UVEB systems, where a solventless medium is cured rapidlyby radiation, are also the source of significant system limitations.

Light curing requires that the coating and/or object must besufficiently transparent in the spectral region of curing, since thepenetration depth and absorption of the curing radiation is essential toachieve rapid and efficient curing. This limits the performanceadditives (fillers, stabilizers, functional additives, and coating aids)that can be added to UVEB systems, since the additives must also fulfillthe requirement of being optically transparent in the curing region ofthe spectrum. While there are some types of addenda that meet thisrequirement, their formulation into UVEB resins can be very difficult,since these systems do not contain diluents or volatile components.

Diluents (solvents and volatiles) act as dispersion aids and carriersthat enable integration of a wide variety of functional additives intopaints and coatings formulations. Diluents give the formulator toolswith which to adjust viscosity and rheology, disperse solids andovercome formulation incompatibilities. These factors, in combinationwith the absorption requirements of UVEB formulations, greatly limit theperformance additives that can be utilized.

The dental industry, primarily due to health concerns, is rapidlytransitioning dental restoratives (e.g., cavity fillings, dentalrestorations) from the conventional mercury-based amalgams to highlyfilled, light curable, polymer-based composites. Polymer-basedcomposites are safer and better match the color and appearance of humantooth enamel, but are often softer, not as strong or as durable as thetraditional metal amalgams. To resolve these problems, manufacturershave developed microfilled polymer composites that have strength,hardness and durability close to that of the conventional amalgams. Toachieve the performance requirements, polymers are highly filled atloadings of 70-80% by weight performance additives. It is generallydesirable that the filling percentage be as high as possible toapproximate the hardness of teeth, however, loadings greater than about80% are very difficult to achieve.

From the patient's perspective, the aesthetic quality of the restorationis extremely important, since teeth are an important part of personalappearance. Matching the aesthetic quality of natural human enamel isdifficult, since teeth, although opaque, have a translucent oropalescent quality that provides luster and visual brilliance. Toachieve these qualities, some dental restorative manufacturers havedeveloped performance additives that are closely matched in refractiveindex to the polymers used to prepare dental restoratives. The moreclosely index-matched the performance additives are to the polymer, thegreater the translucency and aesthetic quality of the restoration.Because the two materials have the same index of refraction, there islittle scatter of light and the resulting restorative compositeresembles natural teeth in optical translucency and appearance. Thisalso has the added benefit that it increases light penetration and thecuring depth of the composite.

There are two types of fillers that are used in dentistry to give highoptical translucency and aesthetic quality. The first is a glass or meltderived filler that is produced by melting a glass composition of knownrefractive index, rapidly cooling or quenching the melt (for exampleinto cold water) into a glass, and then pulverizing the glass to a givenparticle size, usually between about 0.4 and 10.0 microns. This processproduces amorphous, shard-like particles of low surface area, usuallybetween about 1-10 m²/g. A prevalent example of this type of filler isbarium glass.

The second is a microporous filler that is produced from the thermaltreatment of mixtures of colloidal dispersions of oxides, such assilica, zirconia and alumina. The refractive index is controlled throughcontrol of the composition. This process was first developed by Mabie etal., U.S. Pat. Nos. 4,217,264 and 4,306,913, to produce amorphous,microporous mixed oxides of silica and zirconia, and later by RandklevU.S. Pat. No. 4,503,169 to produce crystalline, microporous mixed oxidesof silica, zirconia, and other oxides.

The microporous fillers are highly fused materials consisting of silicaand other oxide particles and, because they are processed at atemperature below the melting temperature of any of the components, theyare porous and have a high surface area. As Randklev pointed out, thesurface area may be as high as 200 m²/g and the average pore volume maybe as high as 40% of the volume of the filler. These microporous fillershave received much attention because of their numerous advantages,including improved finish, gloss, strength, and abrasion resistance.

There is a problem, however, in that for microporous fillers, both theinternal porosity and surface area is high, and it is difficult toachieve high loadings of the porous fillers in dental monomers. Theinternal pores soak up the organic resin, limiting the fraction of resinthat may keep the suspension in a fluid state, and the viscosity risesexponentially making the paste unworkable.

There is an additional problem with modern dental compositerestorations. Modern dental materials contain a liquid, polymerizableresin in the form of monomers, or monomer mixtures, as an essentialcomponent. It is known that, during polymerization, a volume contractiontakes place. The volume contraction is often called shrinkage and isattributable to the development of covalent bonds between the monomermolecules during polymerization, whereby the distance between themolecules is decreased. During the preparation of pre-shaped parts, thepolymerization shrinkage has a very disadvantageous effect on thedimensional stability and the mechanical properties of the moldedbodies. In the case of adhesives and gluing compounds, thepolymerization shrinkage adversely affects the adhesion properties andthe bonding strength, which deteriorates the adhesion betweenrestoration material and the natural tooth substance of dentalmaterials. Voids and cracks may result which become reservoirs forbacteria and encourage the development of secondary caries.

In order to reduce the polymerization shrinkage of dental materials, theindustry has developed pre-polymerized fillers in which a mixture ofinorganic fillers and monomers is polymerized and then ground to thedesired size and then mixed again with monomers to form a flowablemixture that can be molded in tooth restorations. Because a portion ofthe polymer is pre-polymerized, the amount of shrinkage is slightlyreduced. The preparation and use of such fillers, sometimes calledpre-polymers or composite fillers, has been described in the patent andscientific literature.

In the application of composites in dentistry, the greatest problemarises from simultaneously achieving all of the required (or desired)properties of a dental composite. Ideally, a dental composite shouldhave good viscosity and handling before cure so that the dentist maysculpt a restoration matching the adjacent natural teeth. It should havehigh transparency before cure so that the curing-light may penetratedeeply into the composite. After cure, the composite should have highmechanical strength, low volume shrinkage, good translucency (opticalproperties like that of teeth) and high-gloss and abrasion resistance(for aesthetic longevity). In practice, it is exceedingly difficult forthe formulator of dental composites to achieve, simultaneously, all ofthese properties, since many are counter-opposed. For example, largeparticles generally afford dental composites with low-viscosity andhigh-mechanical strength, but unfortunately poor gloss and abrasionresistance, and the exact opposite is true for smaller particles.

U.S. Pat. No. 5,356,951 to Yearn et al. discloses a composition fordental restorative material comprising: (a) a first methacrylate oracrylate monomer having at least one unsaturated double bond, (b) (i) acomposite filler obtained by curing and pulverizing a mixture of a firstglass powder component having a maximum particle diameter of 10 μm orless and a mean particle diameter of 0.1 to 5 μm with a secondmethacrylate or acrylate monomer having at least one unsaturated doublebond, (ii) a second glass powder component having a maximum particlediameter of 10 μm or less and a mean particle diameter of 0.1 to 5 μm,and iii) a fine particle filler having a mean particle diameter of 0.01to 0.04 μm, and a photo-polymerization initiator. The filler describedis a non-porous filler.

U.S. Pat. No. 7,091,258 to Neubert et al. discloses a compositioncomprising: (i) 10 to 80 wt. % organic binder; (ii) 0.01 to 5 wt. %polymerization initiator; (iii) 20 to 90 wt. % particulate compositefiller, comprising a polymerized mixture of organic binder and inorganicfiller, the composite filler particles having an average particle sizeof 20 to 50 μm, each wt. % of (i), (ii), and (iii) relative to the totalmass of the composition; and wherein the composition contains at most 10wt. % composite filler particles having a size of <10 μm, relative tothe total mass of the particulate composite filler in the composition.There is a problem, however, in that the material of Nuebert et al.requires extensive grinding in order to be used as a dental filler, and,at best, a relatively large particle size (20-50 μm) is achieved.

EP 0 983 762 A1 to Katsu discloses an organic-inorganic composite fillerfor use in dentistry. The composite filler is prepared by curing amixture of a particulate filler with an average particle size of 20 nmor less and a methacrylate or acrylate monomer with a viscosity of 60 cPor more and pulverizing the cured mixture. The materials are said to becharacterized by good polishability and good mechanical properties andhave a smoothness and transparency corresponding to the natural tooth.

U.S. Pat. Publ. No. 2013/0005846 to Yamazaki et al. discloses anorganic/inorganic composite filler that contains: inorganic agglomeratedparticles comprising agglomerations of inorganic primary particleshaving a mean diameter between 10 and 1000 nm; an organic resin phasethat covers the surface of each inorganic primary particle and binds theinorganic primary particles to each other; and intra-agglomerate voids,formed between the organic resin phase covering the surface of eachinorganic primary particle, with a pore volume (here, pore refers toholes with diameters between 1 and 500 nm) between 0.01 and 0.30 cm³/gas measured by mercury intrusion porosimetry. There is a problem,however, in that Yamazaki et al. is directed toward bonding or gluingtogether discreet primary particles with a polymer phase and does notprovide high transparency filler materials.

WO2015034881A1 to Bringley et al. discloses composite fillers that canbe loaded at very high weight or volume fractions without negativelyimpacting viscosity, and that reduce the volume contraction or shrinkageof the composite. It further discloses composite fillers that areindex-matched to the monomers or resins into which they are placed,thereby increasing the transparency and aesthetic qualities of thecomposite and uniquely allows for preparation of fillers with very highradiopacity. Still further, Bringley et al discloses porous, mixedparticle inorganic filler materials that are sintered together to forman extensive network of strong inorganic bonds, thus greatly improvingfiller strength. The composite filler does not require pulverization orgrinding and can be used directly in composite formulations. There is aproblem, however, in that the fillers do not provide high-gloss andpolish retention, and Bringley et al. fails to provide a method forachieving high translucency both before and after cure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 contains the full data table, Table 1, from which excerpts areutilized in the Example section of the specification to facilitateunderstanding.

PROBLEM TO BE SOLVED

There is a problem in that the fillers of the prior art cannot meet allof the requirements of high strength, low surface area and high loadingcapacity, low shrinkage, high radiopacity, excellent gloss and wearabrasion, index matching for aesthetic properties, and the eliminationof post-processing steps, such as grinding. There is a problemassociated with the prior art in that the composite fillers are preparedwith a relatively large amount of polymerized organic binder, usually inthe range of 20-30 weight %. This limits the hardness and strength thatcan be achieved. There is a further problem in that thepre-polymerization essentially glues, or binds, the particles togetherinto a mass that must then be pulverized and ground into a filler ofsmaller grain size. This step is time consuming and costly and furtherdegrades the mechanical and aesthetic properties of the composites.Still further, it creates very small particles, often called fines,which increase the viscosity of the mixtures with monomers and limit theloading of the inorganic component. There is a further problem in thatthe composite fillers contain air pockets or voids that degrade theoptical and aesthetic properties of the fillers. There are additionalproblems in that the inorganic components of the composite fillers arenot precisely matched in refractive index with the organic portion,increasing the visual opacity and degrading the aesthetic quality of therestoration.

There is a need for fillers that may be used in dentistry to reduceshrinkage, that allow very high inorganic loading contents withoutcausing a steep rise in viscosity, and good handling and sculptingproperties. There is a need for fillers that do not require costlygrinding procedures and that have adequate strength, hardness andaesthetic qualities. There is a need for composites with exceptionalgloss and gloss retention after abrasion. There is a need for compositesthat have high translucency both before and after cure.

SUMMARY OF THE INVENTION

A composite filler comprising thermally processed porous inorganic mixedparticles of silica and at least one heteroparticle selected from thegroup consisting of zirconia, hafnia, or yttria, wherein the porousinorganic mixed particles are thermally processed at a temperature offrom 650 to 900° C., and a polymer occupying the pores of the thermallyprocessed porous inorganic mixed particles. The present invention alsorelates to a dental restorative comprising a resin and a compositefiller, and optionally other fillers, wherein said resin has arefractive index that increases upon curing, and wherein the opacitiesof the both uncured and cured restorative are less than 45.

ADVANTAGEOUS EFFECT OF THE INVENTION

Embodiments of the present invention include several advantages, not allof which are incorporated in a single embodiment. The variousembodiments of the invention provide composite fillers that can beloaded at very high weight or volume fractions without negativelyimpacting viscosity, and reduce the volume contraction or shrinkage ofthe composite, but also have excellent gloss and gloss retention afterabrasion. Surprisingly, it is found that by controlling the primaryparticle size, sintering and secondary particle size of the fillers,composites can be obtained that, simultaneously, have high loadingcapacity, excellent viscosity and handling, high mechanical strength,good aesthetics, high-gloss and gloss retention and have translucencythat is substantially the same both before and after cure.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a composite filler comprising thermallyprocessed porous inorganic mixed particles of silica and at least oneheteroparticle selected from the group consisting of zirconia, hafnia,or yttria and a polymer occupying the pores of the porous inorganicmixed particles, wherein the porous inorganic mixed particles arethermally processed at a temperature of from 650 to 900° C. to obtain acomposite filler with excellent properties, especially including highgloss retention. Other factors may be included to further enhance theproperties, such as the particle size of the composite filler, the meanparticle diameter of the silica and the proximity of the refractiveindex (RI) of the infused resin to the RI of the porous inorganicparticle.

Terms and Definitions

Median particle diameter or median diameter, as used herein, refers tothe volume-weighted, median particle diameter.

Heteroaggregate, as used herein, refers to a heterocoagulate that hasbeen heated at a temperature sufficient to form strong chemical bondsbetween the distinct colloidal particles, fusing them together, usuallyat a temperature of greater than 600° C. In the present invention, theporous inorganic mixed particle is a heteroaggregate.

Primary particle diameter or median primary particle diameter as usedherein refers to the median particle diameter of the colloids, alsoreferred to as heteroparticles, used to prepare the heteroaggregate,also referred to as the porous inorganic mixed particle.

Secondary particle diameter, median secondary diameter, or mediansecondary particle diameter as used herein refers to the median particlediameter of the heteroaggregate.

Composite Filler, as used herein, refers to fillers comprising both aninorganic and an organic portion.

Composite, as used herein, refers to a polymer or pre-polymer thatcontains at least one inorganic filler, including a composite filler.

Resin, as used herein, refers to a polymerizable mixture of monomers,oligomers or other polymerizable molecules.

Composite Fillers for use in the present invention may be prepared byinfiltrating a variety of different resins or resin mixture withdifferent strength, functional groups, surface energy, refractive index,etc.

The inorganic mixed particle materials of the invention are porous andcontain micropores or microchannels that are substantially open. Porousinorganic filler materials for use in dentistry have been previouslydescribed by Mabie et al., U.S. Pat. Nos. 4,217,264 and 4,306,913 andRandklev, U.S. Pat. No. 4,503,169. These materials of the prior art aretypically produced by sintering the component oxides and/or non-oxidesat high temperature (700-1100° C.). The sintering temperature istypically chosen such that it is below the melting temperature of anycomponent of the mixture. Extensive melting of the components should beavoided since it may lead to particles that are non-porous.

The preferred porous inorganic particles are mixed particles, orheteroaggregates, and comprise silica and at least one particle selectedfrom the group consisting of yttria, zirconia, and hafnia.Heteroaggregates suitable for use in the invention are described in U.S.Pat. No. 8,617,306 to Lambert et al. and in Bringley et al. U.S. Pat.No. 9,017,733B2, each incorporated herein by reference. Otherheteroaggregates useful for the invention comprise silica and at leastone oxide selected from the group consisting of alumina, zinc oxide,titania, zirconia, yttria and rare earth oxides. These oxides arepreferred because of their unique refractive indices and also because oftheir significant radiopacity. Also preferred are heteroaggregatescomprising at least one oxide and a non-oxide filler selected fromhydroxyapatite, fluoroapatite and alkaline earth fluorides. Thesenon-oxide fillers are preferred because they contain calcium, phosphateand fluoride, all of which are known to promote dental health.Non-preferred materials are non-porous fillers such as those derivedfrom melting process such as glasses, and discreet nanoparticle fillersthat are substantially fully densified, although it is possible to usesuch materials as a minor component of the composite filler.

The porous inorganic particles have a median secondary diameter of lessthan 5 microns, more preferably from 2 to 4 microns and most preferablyfrom 2 to 3 microns. These particle size ranges are preferred indentistry because they produce composites that have good mechanicalproperties while also having good wear/abrasion and gloss properties. Inthe practice of the invention, it is possible to use other particles asfillers in relatively small proportions including fumed-silica, bariumor strontium glass, colloidal and precipitated silica fillers, andradiopaque fillers.

The porous inorganic particle materials of the invention are mostpreferably produced by the heterocoagulation of colloids. The colloidsused for preparing the porous inorganic particles of the invention arepreferably selected from aqueous dispersible metal oxide particlesincluding silica, alumina, zirconia, titania, zinc oxide, hafnia, yttriaand rare earth oxides. Most preferably, the colloids are silica,alumina, titania, zirconia, or combinations thereof. Specific examplesinclude colloidal, precipitated or fumed silica, aluminas, such as Al₂O₃and its polymorphs, AlOOH (also known as boehmite), zirconia, ZrO₂ orhydrous zirconia's, rare earth oxides, such as Y₂O₃ and Yb₂O₃, and thebasic carbonates and nitrates of the aforementioned materials. It ispossible to include also other metal oxides, finely ground glasses,and/or metal compounds, such as hydroxides, carbonates, halides,phosphates, nitrates, and the like. Preferred particles that are glassesinclude barium and strontium glasses although, as mentioned above, theyshould be used only as a minor component.

The preferred silica particles are colloidal, precipitated or fumedsilica's having the general formula SiO₂. Silica is used in combinationwith a second colloid to produce a porous mixed oxide inorganicmaterial. This is preferred, because it allows the refractive index ofthe composition to be modulated. It is preferred that the silicacolloids have a particle size of less than 100 nm, and more preferablyfrom about 20 to 80 nm. In a particular embodiment, it is most preferredthat the silica particles are selected from a mixture comprising silicacolloids having median primary particle diameter of 20 and 80 nm.Surprisingly, mixtures of silica colloids of these primary particlesizes have exceptional gloss and gloss stability after abrasion. It ispreferred that the mixture contains at least 20 weight % of each type ofprimary particle.

The heteroparticles of the invention comprise crystalline or amorphousinclusions within the porous inorganic particles and have a medianprimary particle diameter of less than 20 nm. This is preferred, becausecolloids of these dimensions can be mixed to produce the mixednanoparticle aggregates whose refractive index can be modulatedpredictably, based upon the index-weighted, volume fractions of thecomponent nanoparticles.

The colloidal particles before heterocoagulation are preferably stableaqueous colloids. A stable aqueous colloid is one that does not settleor separate from aqueous dispersion for a period of at least one monthor more. It is preferred that the stable aqueous colloids have a meanparticle diameter of between about 1 and 100 nm, more preferably between1 and 50 nm and most preferred between 1 and 25 nm.

The median particle diameters of the composite fillers or the porousinorganic particles of the invention may be characterized by a number ofmethods, or combination of methods, including coulter methods, lightscattering methods, sedimentation methods, optical microscopy andelectron microscopy. Light scattering methods sample a billion or moreparticles and are capable of giving excellent particle statistics.

Light scattering methods may be used to give the percentage of particlesexisting within a given interval of diameter or size, for example, 90%of the particles are below a given value. Light scattering methods canbe used to obtain information regarding mean particle size diameter, themedian particle diameter, the mean number distribution of particles, themean volume distribution of particles, standard deviation of thedistribution(s) and the distribution width for the particles.

In practice of the invention, it is preferred that the particle size isexpressed as the median, volume-weighted particle size. This is thevalue (in microns) at which, by volume, half of the particles are largerand half are smaller.

The heterocoagulation may be accomplished by mixing the selectedcolloids and calcium or phosphorus sources, such as phosphates, in asuitable dispersion medium. The preferred dispersion medium is water.The mixing may be accomplished by using a suitable mixing apparatus,such as a blade or prop-like stirrer, a magnetic stirrer, a staticmixer, in-line mixers, dispersators, or other high shear mixingapparatus. The mixing efficiency of the apparatus is dependent upon thetype of mixing method chosen and the precise geometry and design of themixer. Complete mixing of the two, or more solutions is preferablyaccomplished in less than about 10 seconds, and is more preferablyaccomplished substantially instantaneously.

After heterocoagulation of the particles, a porous inorganic materialwith a given refractive index is produced by drying and thermalprocessing to produce a sintered heteroaggregate. The drying and/orthermal processing may be accomplished in separate steps, or combinedinto a single step. It is most preferred that the dried heterocoagulatedmixed particles are thermally processed at a temperature below themelting point of the mixture, or at least below the melting point of themain component of the mixture. The thermal processing step increases thehomogeneity of the mixture, decreases the apparent surface area, andimportantly, increases the strength of the heteroaggregate. Generally,higher thermal processing temperatures provide stronger materials thathave lower surface areas. However, there is a problem in that if thetemperature is too high it may produce melted aggregates that may havepoor abrasion and gloss properties when employed in composites. This isbecause the hard aggregates may pluck out from the surface of thecomposite leaving behind large voids.

Alternatively, lower thermal processing temperatures lead to materialswith extremely high surface area and poor strength. The precise thermalprocessing characteristics are therefore important to tune theproperties of the composite. It is preferred that the thermal processingtemperature is between about 650 to 900° C., and more preferably fromabout 700-875° C. Surprisingly, it has been found that composite fillersthat are prepared from porous inorganic particles that are sintered orthermally processed at the preferred temperatures have excellent wear,gloss and gloss retention after abrasion.

During the thermal processing step, the particle components fusetogether to form strong, micron-sized heteroaggregates that consist ofmany millions of partially fused nanoparticles. This reduces the surfacearea of the particles and increases their strength. It is preferred thatthe heteroaggregates, after thermal processing, have a specific surfacearea between about 5-200 m²/g and it is more preferred that the surfacearea is controlled to be from about 10-100 m²/g. It is still morepreferred that the surface area is controlled to be from about 40-70m²/g. The reduction in surface area facilitates the integration of thematerials of the invention into polymers, monomers, composites and otherformulations, and also increases the mechanical strength of thecomposites made therefrom. It is further important that the surface areais not reduced to below about 5 m²/g, since low surface area materialshave little porosity and limit the amount of polymer that can be infusedwithin the pores.

After thermal processing, the porous inorganic particles containcrystalline and/or amorphous microdomains or regions. It is preferredthat porous inorganic particles contain at least one crystalline orsemicrystalline phase. It is also highly preferred that the crystallineor semicrystalline phases have microdomains less than about 50 nm andmore preferably less than about 20 nm. The inclusion of suchmicrodomains of crystalline or semicrystalline phases allows therefractive index of the porous inorganic particles to be tuned to agiven value. The smaller the amorphous, crystalline or semicrystallineinclusions, the less scatter of visible light, which allows therefractive index to be tuned to a precise value. Materials of known andnarrow refractive index dispersion are particularly useful in opticalapplications and in applications where the aesthetic quality of adevice, item or article is prized. It is preferred that the refractiveindex of the porous inorganic particles is between about 1.48 and 1.58,most preferably from 1.52 to 1.58. These ranges encompass the refractiveindex range for a wide variety of polymers and monomers that are usefulin optical, medical and coating applications.

The porous inorganic particles of the invention are sintered, alsoreferred to as thermally processed, to produce strong, micron-sizedparticles that are porous. It is preferred that the porosity is producedby high temperature thermal processing, and not by other methods thatproduce only relatively weak particles. The strength of the sinteredparticles of the invention is demonstrated by the fact that, regardlessof the particle size of the colloids used in preparation, the porousinorganic particles are micron-sized and cannot be diminished back intoprimary particles, even with extensive milling or grinding. The porosityserves several functions including improving abrasion and wearresistance of the particles. Porous particles have improved abrasion andwear since they may shear particle-by-particle at the surface of acoating, whereas nonporous materials may pluck out leaving behind avoid. The pores create internal surface, which may soak up monomer(s) bycapillary force and exclude monomer from the external surfaces of theparticle. The pores are substantially open and accessible by diffusionto small molecules and/or oligomers. It is preferred that the porousinorganic particles are substantially free of closed pores since closedpores are not accessible by diffusion and thus prevent polymerizationwithin the pores, and because closed pores reduce transparency. It ispreferred that the pores constitute approximately 10-70%, and morepreferably 25-50% of the volume of the particle.

In the practice of the invention, an organic material, typically apre-polymer, is infused within the pores of the porous inorganicmaterial and polymerized therein to produce a composite filler. Thepores are infused with monomers, oligomers and/or polymer precursors(collectively referred to as resins) that are subsequently polymerizedwithin the pores, such that the pores are substantially filled withpolymer.

It is preferred that the composite filler is at least 70 percent byweight, preferably 80 percent by weight, and more preferably greaterthan 84% by weight, porous inorganic particles. Conversely, it ispreferred that the polymer occupying the pores of the porous inorganicparticles comprises a weight percent from about 8 to 16%. In practice ofthe invention it is important to match, as best as possible, the polymervolume with the pore volume. This insures that the majority of theresin, and after polymerization the polymer, is absorbed within thepores. Excessive polymer concentrations may lead to bonding or gluing ofparticles together, thus necessitating milling or grinding steps todiminish the particle size. It is preferred that the median secondaryparticle diameter of the composite filler, after polymerization, is notgreater than 2-times the median secondary particle diameter of theporous particles.

The composite filler of the invention has low surface area and thereforecan be loaded at very high solids concentrations within resins. The lowsurface area is brought about by filling the pores of the porousinorganic particles with polymer. It is preferred that all of the poresin the composite filler are completely filled and the composite filleris substantially free of voids.

Resins containing the composite filler, when cured, produce compositeswith high strength, gloss, wear and abrasion characteristics, and lowcuring shrinkage. Most importantly, the composite filler, when properlymatched to a resin system, and/or a cured resin system (i.e., apolymer), may produce composites that have exceptional aestheticqualities such as high transparency and translucency. The transparencyof a composite or article can be measured in a variety of ways, the mostcommon of which is the fluid immersion method wherein a filler isdispersed within fluids of known refractive index and relative lighttransmission of the resulting dispersion is measured. The maximum lighttransmission corresponds to a matching of the refractive index betweenthe filler and the fluid, and so provides a method for determining therefractive index of the filler. This provides the maximum transparencywhen, in practice, the composite fillers are dispersed within a resinand the resin cured to produce an article.

In a simple form, transparency refers to the ability to see through anobject (such as the case for a window) and to recognize and discernobjects on the other side. The transparency of articles prepared usingthe inventive compositions will be dependent upon the precise indexmatch, the loading of the composition within the resin, and thethickness of the article produced. Herein, the transparency is measuredby way of a Transparency Index, described in the Description of TestingMaterials: Optical Measurements in the Experimental section, whereinhigher index indicates greater transparency. Transparency indices of 12or greater indicate transparencies approaching window glass, whereas anindex of about 8-11 represents a slightly scattering (translucent)medium, and indices of 7 and below indicate increasingly opaquematerials. It is preferred that the transparency of the composite filleris greater than 8, it is more preferred that it is greater than 10.

Management of Refractive Index. In practice of the invention, the actualchoice of resins and refractive indices for all components is quitecomplex. The complexity arises from the fact that, although most resinsystems have a refractive index between about 1.44 and 1.60, theirrefractive index typically increases upon polymerization, usually byabout 0.02-0.03 units. In the invention, it is preferred that both theporous inorganic particles and the composite filler, have a refractiveindex between 1.48 and 1.58, more preferably between 1.52 and 1.58.

However, in practice of the invention, it is possible to tune or adjustthe refractive index of the composite filler, by proper choice of thecomponents. When the porous inorganic particles are infused with apolymer having a lower refractive index, the refractive index of thecorresponding composite filler is reduced. Accordingly, when the porousinorganic particles are infused with a polymer having a higherrefractive index, the refractive index of the corresponding compositefiller is increased. In order to facilitate the adjustment of refractiveindex, it is preferred that the difference between the refractiveindices of the polymer and the porous inorganic particles is greaterthan 0.015.

Shade matching the restoration before and after cure. In a preferredembodiment, the composite fillers of the invention are employed indental restorations. The object of dental restorations is tore-construct teeth to their original form, function and appearance, forexample, after caries or trauma to the jaw. There are multiplechallenges in restorative dental technology, as the dentist must apply,form and sculpt a restoration that matches tooth form, function(strength), shade and appearance. In practice, after the restoration isprepared, it is cured in place by using a high-energy light-wand in theblue or UV range of the light spectrum. There is a problem that arisesas a result of the fact that the refractive index of the resin changesduring curing. It is essential that the restoration be translucent tothe curing radiation, so that the light may penetrate deeply andproperly cure the inner portion (i.e., the backside adjacent to theremaining tooth structure). However, because the refractive index of theresin portion changes during curing, the optical or aesthetic appearanceof the restoration also changes, and can lead to a shift in color of therestorations such that it no longer matches the adjacent natural teeth.This is especially true if the opacity (i.e., the opposite oftransparency) of the restoration changes significantly during cure. Inthe prior art, it is most typically practiced that the filler and resin(before polymerization) comprising the composite have the samerefractive index. Therefore, the optical properties of the compositechange during curing, potentially leading to a color shift. The authorsof the present invention have discovered, surprisingly, that if thepolymerized resin portion of the composite filler has a refractive indexthat is significantly greater than the filler, it shifts the effectiverefractive index of the composite filler to a higher value. Further, itis discovered that dental composites prepared from such compositefillers have optical properties that are closely matched before andafter cure, and avoid the shade mismatch dilemma discussed above.

The invention described herein solves this problem as it provides adental restorative comprising a resin and a composite filler, andoptionally other fillers, wherein the resin has a refractive index thatincreases upon curing, and wherein the opacities of the both uncured andcured restorative are less than 45. Low opacity values are preferredbecause they provide for good optical translucency, affordingrestorations with luster and optical brilliance; and also because theyhave good light penetration ensuring that the restoration is adequatelycured. It is further preferred that the opacity, both before and aftercure, is less than 40. It is preferred that the difference between theuncured and cured opacities is less than 15. This is preferred becauseit minimizes color shift of the restoration during curing.

In practice of the invention, the restorative must be strong anddurable. The human oral cavity is an extremely challenging environmentas it is constantly wet, subject to debris and bacteria, and subject tomastication forces over many decades. To increase strength anddurability it is preferred that the composite filler contains at leastone monomer that is a trifunctional, tetrafunctional or a greaterfunctional monomer. This is preferred because it allows for greatercross-linking or “anchoring” of the composite filler into the dentalrestorative during cure.

In order to facilitate the integration of the compositions of theinvention into polymers, monomers, composites or other formulations, itmay be necessary to functionalize the surfaces of the porous inorganicparticles with surface agents, for example, surfactants, coating aids,coupling agents, or the like. This step may be accomplished before, orafter, the infusion and polymerization processes. It is preferred thatit is done before the infusion and polymerization process. It ispreferred that the particles have their surfaces functionalized bysilane coupling agents, or hydrolyzed precursors of silane couplingagents having the general formula:R_(a)R′_(b)Si(OR″)_(4−(a+b)),where a and b are integers from 1 to 3, (a+b) is less than or equal to3, R and R′ are organic groups having from 1-30 carbon atoms and R″ isH, or an organic group having from 1 to 6 carbon atoms.

Alternatively, the silane coupling agent may have the general formula:R_(a)Si(X)_(4−a),where a and R is as defined above and X is a halogen, Cl, Br or I.Specific examples of silane coupling agents useful for practice of theinvention include but are not limited to3-mercaptopropyl(trimethoxy)silane,3-mercaptopropylmethyl(diethoxy)silane,methacryloxypropyl(trimethoxy)silane,2-[methoxy(polyethyleneoxy)propyl](trichloro)silane,2-[methoxy(polyethyleneoxy)propyl](trimethoxy)silane,octyl(trimethoxy)silane, octadecyl(trimethoxy)silane,3-isocyanatopropyldimethylchlorosilane,3-isocyanatopropyl(triethoxy)silane,Bis(2-hydroxyethyl)-3-aminopropyltriethoxysilane,aminopropylsilanetriol, 3-aminopropyl(triethoxy)silane,3-aminopropyl(trimethoxy)silane,N-(2-aminoethyl)-3-aminopropylsilanetriol,N-(2-aminoethyl)-3-aminopropyl(trimethoxy)silane,N-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane,isopropyl(trimethoxy)silane, (3-glycidoxypropyl)methyldimethoxysilane,tetradecyldimethyl(3-trimethoxysilylpropyl)ammonium chloride,(3-trimethoxysilylpropyl)diethylenetriamine andoctadecyldimethyl(3-ammonium)propyl(trimethoxy)silane.

To initiate the surface reaction, the particles and the surface agent(s)are mixed together in a high shear mixing zone within a dispersionmedium. It is preferred that the dispersion medium is water, but othersolvents or liquids may also be used.

In the preparation of the composite filler of the invention, the porousinorganic nanoparticle material is mixed with a resin or pre-polymer,usually within a solvent or medium in which the resin is soluble.Suitable media are any liquid in which the resin or pre-polymer issoluble, but preferred media are water and or organic solvent, such asacetone, methanol, ethanol, isopropanol, ether or other volatile organicsolvents. A polymerization initiator and/or accelerator is then added tothe mixture. Initiators and accelerates generally initiatepolymerization only after a stimulus is applied such as heat, light orother radiation. After the mixture is homogeneously mixed, the solventis then removed by vacuum distillation, or another evaporative process.This reduces the mixture to a free flowing powder.

The resin or pre-polymer portion of the powder is then polymerized byapplication of light, heat or other known means to initiatepolymerization. It is preferred that the organic resin or pre-polymermaterial is polymerized substantially within the pores and not on thesurface, or outside of, the porous inorganic nanoparticle material.Although some polymerization outside of the pores is difficult, if notimpossible in practice to prevent, surprisingly, at the appropriateloadings, substantially all of the resin is polymerized within thepores. In another embodiment of the invention, the composite fillerparticles of the invention are dispersed within a matrix.

The matrix may comprise at least one fluid, polymer, oligomer, monomeror combinations thereof. It is preferred that the inventive compositionsare dispersed within the polymer, oligomer, or monomer matrix at aloading of 1-80% by weight. It is preferred that the polymer, oligomerand/or monomer(s) are thermal or light curable. Useful examples ofpolymers for the matrix are acrylate-functional materials,methacrylate-functional materials, epoxy-functional materials,vinyl-functional materials, and mixtures thereof. Typically, thehardenable resin is made from one or more matrix forming oligomers,monomers, polymers, or blend thereof. Also useful are urethanes,fluoropolymers, siloxanes and latex polymers.

In certain embodiments, the inventive materials are used in dentalapplications or in orthopedic, or other in vivo, applications. It ispreferred that the composite filler is a dental composite filler. It ispreferred that the composite is dispersed in a light polymerizable resinmatrix. Polymerizable matrix materials suitable for use in theseapplications include hardenable organic materials having sufficientstrength, hydrolytic stability, and nontoxicity to render them suitablefor use in the oral or in vivo environment. Examples of such materialsinclude acrylates, methacrylates, urethanes, carbamoylisocyanurates,epoxies, and mixtures and derivatives thereof. One class of preferredhardenable materials includes materials having polymerizable componentswith free radically active functional groups. Examples of such materialsinclude monomers having one or more ethylenically unsaturated group,oligomers having one or more ethylenically unsaturated group, polymershaving one or more ethylenically unsaturated group, and combinationsthereof.

In the class of hardenable matrix resins having free radically activefunctional groups, suitable polymerizable components for use in theinvention contain at least one ethylenically unsaturated bond, and arecapable of undergoing addition polymerization. Such free radicallyethylenically unsaturated compounds include, for example, mono-, di- orpoly-(meth)acrylates (i.e., acrylates and methacrylates), such as,methyl (meth)acrylate, ethyl acrylate, isopropyl methacrylate, n-hexylacrylate, stearyl acrylate, allyl acrylate, glycerol triacrylate,ethyleneglycol diacrylate, diethyleneglycol diacrylate,triethyleneglycol dimethacrylate, 1,3-propanediol di(meth)acrylate,trimethylolpropane triacrylate, 1,2,4-butanetriol trimethacrylate,1,4-cyclohexanediol diacrylate, pentaerythritol tetra(meth)acrylate,sorbitol hexacrylate, tetrahydrofurfuryl (meth)acrylate,bis[1-(2-acryloxy)]-p-ethoxyphenyldimethylmethane,bis[1-(3-acryloxy-2-hydroxy)]-p-propoxyphenyldimethylmethane,ethoxylated bisphenol A di(meth)acrylate, andtrishydroxyethyl-isocyanurate trimethacrylate; (meth)acrylamides (i.e.,acrylamides and methacrylamides), such as (meth)acrylamide, methylenebis-(meth)acrylamide, and diacetone (meth)acrylamide; urethane(meth)acrylates and the bis-(meth)acrylates of polyethylene glycols.Other suitable free radically polymerizable compounds includesiloxane-functional (meth)acrylates and fluoropolymer functional(meth)acrylates. Mixtures of two or more free radically polymerizablecompounds can be used, if desired.

Examples of other useful matrix polymers include natural and syntheticbiopolymers, such as peptides, proteins, gelatin, poly(lactic acid),poly(glycolic acid), poly(caprolactone), chitosan and its derivatives,alginates and the like.

EXAMPLES

The following examples are provided to illustrate the invention.

Materials

All material concentrations are given as weight to weight percentagesunless otherwise noted.

NALCO 2327® is a colloidal dispersion of silica in water, the meansilica particle diameter is 20 nm and the solids concentration 40.0%.

NALCO 2329® is a colloidal dispersion of silica in water, the meansilica particle diameter is 75-100 nm and the solids concentration40.0%.

NALCO DVSN004® is a colloidal dispersion of silica in water, the meansilica particle diameter is 40 nm and the solids concentration 40.0%.

Zirconyl Acetate® is a colloidal zirconia dispersion sold by NyacolNanotechnologies with a mean particle diameter of 5-10 nm.

2,2′-Azobis(2-methylpropionitrile) (AlBN) is a thermal polymerizationinitiator purchased from Aldrich Chemical Company.

SR541, SR238B, SR351, hexanediol diacrylate, Trimethylolpropanetriacrylate, urethane dimethacrylate (UDMA), SR101 and triethyleneglycol dimethacrylate (TEGDMA) are polymerizable methacrylate andacrylate monomers purchased from Sartomer USA, LLC.

Ethioxylated Bis Phenol A Dimethacrylate (EBPADMA) was purchased froEsstech Inc.

Resin mixture A consisted of a mixture of the monomers SR541, EBPADMA,SR238B and SR351 in a weight ratio of 50:20:15:15, respectively. Thecalculated refractive index of the resin mixture was 1.507; polymerizedResin mixture A has a refractive index of 1.532).

Description of Testing Methods.

Calculation of Refractive Indices.

Refractive indices (η_(tot)) were estimated for all compositions usingthe relationship given in equation 1.η_(tot)=(n ₁ V ₁ +n ₂ V ₂ +n ₃ V ₃)/V _(tot)  (1)where η₁, η₂ and η₃ are the refractive indices of the individualcomponents and V₁, V₂ and V₃ are the respective volume fractions of thatcomponent. The refractive indices used were the reported values; (1.46for SiO₂, 1.675 and 2.115 for zirconia). The volumes for each phase arecalculated based on the weight percentages and densities of thecomponents. Monomer indices and densities were taken from themanufacturers published literature. For polymerized samples, the polymerindex is estimated to increase by 0.025 units from that of the resinsystem; this increase is consistent with reported literature values fordental resins.Optical Measurements

The refractive index match and relative transparency in dental monomerswas determined by making mixtures of the mixed oxide in a monomer ofknown refractive index at 35 wt. %. The monomers were purchased fromSartomer Chemical or Esstech Inc. and are methacrylate monomers commonlyused in dental restoratives. The mixtures were sonicated to remove airbubbles, and 3.08 g of the mixtures were added into a glass vials to adepth of about 7.0 mm. The mixtures were placed on a light box and aseries of optical targets were viewed by looking through the thicknessof the sample. The mixtures were given a relative transparency scorecorresponding to the smallest font feature discernable (clearly visibleand readable). Font sizes varied from 26-point to 2-point. For example,a rating of 1 indicates that only a 26-point font is readable, a scoreof 5 refers to readability of 18-point font or larger, 10=8-point and13=2-point or larger. This simple qualitative method of determiningtransparency has an estimated accuracy of ±a score of 0.5. This methodof ranking the relative transparency of the mixtures was validated usingtransmission spectrophotometry. The mixtures described above weremeasured on a Perkin Elmer Lambda 20 spectrometer at a thickness of 1.0mm in a borosilicate glass slide cell (empty cell used as reference).Transparency was determined as the mean % transmission between 500 to600 nm. Samples were approximately 1 cm from the detector. Refractiveindices of the powders were approximated by placing the powders in aseries of fluids of known refractive index and noting the highesttransparency.

General Procedure for Producing Composite Fillers:

Preparation of Composite Filler (C1): Into a 20 L reactor containing2,076.1 g of zirconyl acetate (Nyacol Nanotechnologies, 20.0% zirconiasolids) that was stirred with a prop-like mixer spinning at 2000 rpm,was added 4,104.6 g of colloidal silica (NALCO 2327; 40.0% silicasolids) at a rate of 90.0 g/min. After addition, the reaction mixturewas allowed to stir for 1 hour. After preparation, the product was driedin a forced air oven at 110° C. The solid obtained was milled with 9 mmalumina beads for 3 hours and the resulting fine powder was fired in aprogrammable furnace at 966° C. for 3 hours and allowed to cool. To450.0 g of the filler thus obtained was added 400.0 g acetone, 27.0 g ofgamma-methacryloxypropyl(trimethoxy)silane and 8.1 g 0.1 N acetic acidand the contents stirred for 16 hours. After this time was added 74.25 gof resin mixture A and 0.74 g of AIBN. The solvent was then removedunder vacuum at 45° C. and the resin was polymerized by heating thepowder at 110° C. for 4 hours under nitrogen gas. This procedure yieldeda composite that contained 14.2% polymer by weight and a mean particlediameter of 6.0 microns. The corresponding fillers of the invention wereprepared by modification of this general procedure including silicacolloid primary particle diameter, milling time, firing temperature,polymer weight percent and final median particle diameter as indicatedin Table 1.

Procedure for Producing Composite Filler Used in Example 12.

The composite filler used in Example 12 was prepared identically to thatof Example 3, except that Resin 1 (refractive index 1.525; polymerizedrefractive index 1.550) was substituted for resin mixture A.

Preparation and Evaluation of Composites.

Composite examples, including one comparative sample (C1) and 11 testsamples, were prepared to illustrate the benefits of including CompositeFillers described above in dental composites compromising alight-curable resin base material that include commercially availablemonomers containing methacrylate groups and a hydrophobic fumed-silica,specifically CaboSil TS530 (available commercially from Cabot. Inc.).

TABLE 2 Resin Composition Resin 1 Resin 2 (wt %) (wt %) BisGMA(Bisphenol A Diglycidyl  9.9% 31.6% ether dimethacrylate) TriethyleneGlycol Dimethacrylate  4.9% 13.7% Ethoxylated Bisphenal A dimethacrylate24.7% 27.8% (no. of ethoxy groups = 3.5) Ethoxylated Bisphenal Adimethacrylate 59.2% — (no. of ethoxy groups = 6) Urethanedimethacrylate — 25.8% 2-hydroxy-4-methoxybenzophenone  0.5%  0.6% BHT(2-hydroxy-4-methoxy Benzophenone  0.1%  0.0% butylated hydroxytoluene)Camphorquinone  0.2%  0.1% Ethyl-4-dimethylamino benzoate  0.5%  0.4%Total  100%  100% Refractive Index (before polymerization) 1.525 1.512Table 2 lists the components of the resins that were used for ComparisonExample 1 and Examples 2-12.Comparison Example 1. Twenty-one grams composite filler (C1), whosepreparation is given above, and 0.6 g fumed-silica TS530, are addedportion by portion to 8.4 g of Resin 1 while mixing with a high speedmixer. After thoroughly mixing, the composite paste was de-aerated underattenuated oxygen pressure.Examples 2-12. Examples 2-12 were prepared in an identical manner asthat of Comparison Example 1, except that the composite filler wasmodified as given in Table 1, and substituted for composite filler C1.Composite Test MethodFlexural Strength and Flexural Modulus.

For the three-point flexural strength test, 6 bar-shaped specimens werefabricated from each comparison example or example, according toISO4049. The composites were packed inside a stainless steel mold (25mm×2 mm×2 mm) and covered with a piece of mylar sheet to extrude excessmaterials. The specimens were polymerized for 120 s on each side in alight curing box. Specimens were soaked in water at 37° C. for 24 h andafter removed from the mold. Specimens then were tested using athree-point bending device test in a universal testing machine at acrosshead speed of 0.5 mm/min until fracture.

Sample Preparation and Polish.

Composite was first loaded in a stainless steel mold (1.5 mm thick, 3.8cm in diameter), pressed down with a glass block with Mylar film on topof the composite, and then cured with following steps: Spot curecomposite for 60 seconds at each location in overlapping increments.Start in the middle and work the curing light around the middle untilthe complete composite surface is exposed. Remove the specimen andrepeat the light cure on the underside.

The cured composite specimen was polished with 240-grit silicon carbidepaper followed by 600-grit on both surfaces. Using a polishing rubberwheel, the specimen was polished in one uniform direction for 60 secondsensuring the entire surface had been covered, The specimen was rotated45 degrees and the polish repeated with a rubber wheel until all fourdirections had been polished. The specimen was then rinsed with water toremove debris. Similar polish steps were performed by replacing thepolishing rubber wheel with a Felt wheel (on-layer medium) and polishingpaste (Diamond 800_654) first then, same polish steps were followed withFelt wheel (on-layer medium) and polishing paste (Superfine Diamond800_656), last polish step were done similarly using Felt wheel (threelayer medium buffer wheel. After conditioning the specimen in water at37° C. for seven days, Gloss retention (initial) was measured on theappropriately dried specimen with the BYK Gardner gloss instrument setat 60°. A measurement was performed and the specimen was rotated 45degrees before taking another measurement. In all, four measurements ofgloss were taken at positions 12:00, 3:00, 6:00 and at 9:00 and theaverage of the four measurements was computed.

Gloss Retention after Toothbrush Wear.

The conditioned polished specimen was fit into a turn-table with areservoir to accommodate a diluted toothpaste slurry. An Oral B electrictooth brush was fitted into another turning fixture with the toothbrushhead positioned at the center of the specimen. The specimen on the turntable was rotated at 165 rpm, while the toothbrush moved across 2 cm ofthe specimen surface in a linear back-forth motion with frequency about9-lap/minute. A constant hanging weight of 300 g was applied to thetooth brush turning fixture 10 cm away to the center of the turningtable to assure good contact between the toothbrush head and thespecimen surface.

Meanwhile the toothbrush head vibrated per Oral B electric toothbrushmanufacturer specification. Gloss retention of specimen was measuredafter 1-hour and 2-hour toothbrush wear with the BYK Gardner glossinstrument set at 60° following the same procedure described previously,the average of four gloss values was reported.

The data of Table 1 of FIG. 1 show multiple benefits. The gloss of thedental composites of the invention are dramatically improved over thecomparison composite that has a median secondary particle diameter of6.0 microns, as illustrated by this Excerpt A of Table 1.

Excerpt A C1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 Ex. 9 Ex. 10 Ex.11 Ex. 12 Composite Filler Properties 2º particle diameter 6.0 4.0 2.62.6 3.2 4.3 4.8 4.6 3.8 3.8 3.8 2.9 (microns) Composite Properties Glossretention 51.8 90.3 90.6 90 89.3 89.1 84.9 82.5 88.0 87.2 89.8 89.8(initial)

The data further show that composite fillers prepared at a lowersintering temperature have, quite surprisingly, very significantlyimproved gloss retention after abrasion, as illustrated by this ExcerptB of Table 1.

Excerpt B C1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 Ex. 9 Ex. 10 Ex.11 Ex. 12 Composite Filler Properties Firing Temperature, 966 966 966866 866 766 816 866 766 816 866 866 ° C. Composite Properties Glossretention 51.8 90.3 90 90.6 89.3 89.1 84.9 82.5 88.0 87.2 89.8 89.8(initial) Gloss retention 45.8 54.2 50.8 73.7 61.9 83.1 81.2 79.7 80.778.1 71.2 71.2 (after 1 h) Gloss retention 45.3 57.5 59.9 74.8 70.3 82.780.9 82.1 78.6 79.7 70.9 70.9 (after 4 h)

The data of Table 1 also show that gloss retention improves withincreasing silica primary particle diameter, as illustrated by thisExcerpt C of Table 1. This is an important result since larger primaryparticles have lower surface area and allow the dental compositeformulator greater freedom in adapting the paste viscosity.

Excerpt C Ex. 6 Ex. 7 Ex. 8 Ex. 9 Ex. 10 Ex. 11 Composite FillerProperties Silica 1º particle 80 80 80 20 & 80* 20 & 80* 20 & 80*diameter (nm) Composite Properties Gloss retention 89.1 84.9 82.5 88.087.2 89.8 (initial) Gloss retention 83.1 81.2 79.7 80.7 78.1 71.2 (after1 h) Gloss retention 82.7 80.9 82.1 78.6 79.7 70.9 (after 4 h) *Silicaprimary particles were chosen from Nalco 2327 and Nalco 2329 at a 50:50weight ratio.

Further the data of Table 1 show that if the resin that comprises theorganic portion (polymer) has a refractive index that is close to thatof the porous inorganic particles, then the opacity of the resultingcomposite is low. A low opacity is advantageous since it gives the toothrestoration translucency and optical brilliance and allows the dentistto better match natural tooth appearance and shade. Still further, theopacity remains about the same both before and after cure, and thereforeavoids changes in shade and appearance after the tooth restoration isfully cured, as illustrated by this Excerpt D of Table 1.

Excerpt D C1 Ex. 3 Ex. 12 Composite Filler Properties FiringTemperature, ° C. 966 866 866 RI** porous inorganic 1.523 1.523 1.523particles RI** polymerized organic 1.532 1.532 1.550 portion (resin)Composite Properties Opacity (before cure) 34.5 24.4 27.8 Opacity (aftercure) 52.6 51.0 38.9 **RI = refractive index

The invention claimed is:
 1. A composite composition comprising: afiller comprising inorganic mixed particles of silica and zirconia,thermally processed and containing pores; and a polymer occupying thepores of the thermally processed inorganic mixed particles of silica andzirconia; and a resin; wherein the composite composition exhibits anaverage gloss retention after 1 hour of abrasion of at least 61.9 glossunits.
 2. The composite composition of claim 1, wherein the silicaparticles of the filler have a median primary particle diameter of lessthan 100 nm.
 3. The composite composition of claim 1, wherein: thesilica particles of the filler are chosen from a mixture comprisingsilica colloids of a first type and a second type; the first type of thesilica colloids has a median primary particle diameter of 20 nm and ispresent in the mixture at a concentration of at least 20 weight percent;and the second type of the silica colloids has a median primary particlediameter of 80 nm and is present in the mixture at a concentration of atleast 20 weight percent.
 4. The composite composition of claim 1,wherein the polymer occupying the pores of the thermally processedinorganic mixed particles of silica and zirconia is present in thefiller at a weight percent from 8 to
 16. 5. The composite composition ofclaim 1, wherein the zirconia particles are present in the thermallyprocessed inorganic mixed particles at a concentration greater than 25percent by weight.
 6. The composite composition of claim 1, wherein thethermally processed inorganic mixed particles of silica and zirconiahave a particle size distribution width of about 4 microns.
 7. Thecomposite composition of claim 1, wherein the resin comprises a freeradically ethylenically unsaturated compound.
 8. The compositecomposition of claim 1, further comprising fumed silica; wherein thefiller, the resin, and the fumed silica are present in the compositecomposition in approximately the proportion 35:14:1.
 9. The compositecomposition of claim 1, wherein the thermally processed inorganic mixedparticles of silica and zirconia have been thermally processed at atemperature of from 700° C. to 875° C.
 10. The composite composition ofclaim 9, wherein: the thermally processed inorganic mixed particles ofsilica and zirconia have been thermally processed at a temperature ofabout 866° C.; and the thermally processed inorganic mixed particles ofsilica and zirconia have a secondary particle diameter of from 2 micronsto 4 microns.
 11. A composite composition comprising: a fillercomprising inorganic mixed particles of silica and zirconia, thermallyprocessed and containing pores; and a polymer occupying the pores of thethermally processed inorganic mixed particles of silica and zirconia;and a resin; wherein the composite composition exhibits an average glossretention after 4 hours of abrasion of at least 70.3 gloss units. 12.The composite composition of claim 11, wherein the silica particles ofthe filler have a median primary particle diameter of less than 100 nm.13. The composite composition of claim 11, wherein: the silica particlesof the filler are chosen from a mixture comprising silica colloids of afirst type and a second type; the first type of the silica colloids hasa median primary particle diameter of 20 nm and is present in themixture at a concentration of at least 20 weight percent; and the secondtype of the silica colloids has a median primary particle diameter of 80nm and is present in the mixture at a concentration of at least 20weight percent.
 14. The composite composition of claim 11, wherein thepolymer occupying the pores of the thermally processed inorganic mixedparticles of silica and zirconia is present in the filler at a weightpercent from 8 to
 16. 15. The composite composition of claim 11, whereinthe zirconia particles are present in the thermally processed inorganicmixed particles at a concentration greater than 25 percent by weight.16. The composite composition of claim 11, wherein the thermallyprocessed inorganic mixed particles of silica and zirconia have aparticle size distribution width of about 4 microns.
 17. The compositecomposition of claim 11, wherein the resin comprises a free radicallyethylenically unsaturated compound.
 18. The composite composition ofclaim 11, further comprising fumed silica; wherein the filler, theresin, and the fumed silica are present in the composite composition inapproximately the proportion 35:14:1.
 19. The composite composition ofclaim 11, wherein the thermally processed inorganic mixed particles ofsilica and zirconia have been thermally processed at a temperature offrom 700° C. to 875° C.
 20. The composite composition of claim 19,wherein: the thermally processed inorganic mixed particles of silica andzirconia have been thermally processed at a temperature of about 866°C.; and the thermally processed inorganic mixed particles of silica andzirconia have a secondary particle diameter of from 2 microns to 4microns.